System and method for monitoring a dosage pump in a dialysis machine

ABSTRACT

Apparatus is disclosed for monitoring the flow of a fluid through a dosage pump in a dialyzer including an auxiliary pump disposed between a fluid source for the dosage pump and the dosage pump itself, a slave chamber between the auxiliary pump and the dosage pump, the slave chamber including a level detector for detecting a predetermined level of the fluid in the salve chamber and emitting a signal when the level of the fluid is below the predetermined level, and a controller for activating the auxiliary pump after the level indicator emits the signal such that the slave chamber is refilled with the fluid by the auxiliary pump after the suction stroke of the dosage pump has drawn the fluid into the dosage pump and caused the level indicator to emit the signal. Methods for monitoring the flow of a fluid through the dosage pump are also disclosed.

FIELD OF THE INVENTION

The present invention relates to a method for monitoring a dosage pumpintended for pumping a concentrate to be diluted in water. Moreparticularly, the present invention relates to a monitoring system for adosage pump for pumping a concentrate fluid at a very low flow rate,preferably in connection with a dialysis machine.

BACKGROUND OF THE INVENTION

European Patent Application No. 278,100 describes a dialysis machine inwhich the present invention may be utilized. That dialysis machinecomprises a preparation unit for a dialysis fluid in which preparationof the dialysis fluid takes place on-line, starting from concentrates inpowder form located in separate cartridges.

Normally, a dialysis machine comprises two portions, a first bloodhandling portion for feedings blood from a patient through anextracorporeal fluid circuit comprising a dialyzer, and a seconddialysis fluid handling portion for preparing a dialysis fluid andtransporting it through the dialyzer, and then to a drain. The dialyzercomprises a semi-permeable membrane separating the dialyzer into a bloodcontaining portion and a dialysis fluid containing portion. Transport ofmolecules, ions, substances and water takes place across the membranefor conditioning the blood to replace the function of the kidneys.

The dialysis fluid normally has a composition which substantiallymatches that of the patient's blood plasma, with certain modifications.In addition to water, a dialysis fluid normally comprises the followingsubstances in ionic form: sodium, bicarbonate, potassium, magnesium,calcium, chloride and acetate. The pH value of the fluid is adjusted tobetween about 7.1 and 7.4. In addition, the fluid may comprise glucoseand other substances.

Two ions are present in large quantities in the dialysis fluid, namelysodium and bicarbonate.

European Patent Application No. 278,100 describes the preparation of adialysis fluid in which these two ions are obtained on-line from powdercartridges containing sodium bicarbonate and sodium chloride,respectively, in the form of a dry powder or granules. Water is passedthrough the cartridges and substantially saturated fluids of sodiumbicarbonate and sodium chloride, respectively, exit the cartridges. Twodosage pumps ensure that the correct quantity of concentrate fluid isfed to a main conduit comprising clean water obtained from a reverseosmosis unit.

The dialysis fluid normally comprises about 35 mmol/l of bicarbonate andabout 140 mmol/l of sodium. In total, about 120 liters of dialysis fluidis consumed during one treatment, which normally lasts for four hoursand takes place three times a week.

Furthermore, the dialysis fluid contains magnesium, potassium, calcium,acetic acid and glucose in suitable quantities. In the dialysis machineaccording to European Patent Application No. 278,100, these othercomponents are obtained from an ionic bag. Since these substances have arelatively low concentration in the prepared dialysis fluid, thecontents of the ionic bag can be very concentrated, in the ratio ofabout 1:200 to 1:500, whereby the volume of the bag is small, about ½liter.

The dosing of the contents of the ionic bag is performed using a dosagepump. The dosage pump feeds the contents of the ionic bag to the mainconduit in the dialysis machine with a flow rate of about 1 ml/min.

A dialysis machine further comprises a supervisory system whichsupervises or monitors vital operations of the dialysis machine. Amalfunction of such vital operations could result in the patient notobtaining adequate treatment, becoming ill, or being harmed or evendying.

One operation which must be monitored is the dosage pump of the ionicbag. Too high a dosage of the contents in the ionic bag could lead toheart failure, while too low a dosage could lead to other symptoms.

It is not simple to monitor such a low flow rate as that which passesfrom the ionic bag; i.e., the order of about 1 ml/min. A large deviationmust be able to be noted quickly enough for suitable corrective measuresto be undertaken, at least within one minute, and preferably within tenseconds. The accuracy must be high and at least within the range ofabout +/−5%.

The contents of the ionic bag comprise salts having a high ionicstrength. Mechanical flow measurement devices run the risk of jamming ifsalt crystals are precipitated, for which there is a great risk.

It is previously known to measure such small flows using thermal flowsensors (see, for example, German Patent Application No. 4,127,675).These sensors are, however, greatly influenced by a change in theambient temperature, and false alarms may easily be emitted. A dialysismachine must operate equally well at temperatures of about 20° C., aswell as at ambient temperatures of 35° C., which may be the case incertain countries. In addition, large temperature differences andtemperature changes arise internally in a dialysis machine, for exampleduring and shortly after heat sterilisation, which may also lead toproblems. In certain cases, a thermal flow detector must be calibratedfor different types of fluids because of different densities and heatcapacities dependent on the concentration of the constituent substances.

One object of the present invention is to provide a system and a methodfor monitoring a dosage pump intended for low flow rates, in the orderof about 1 ml/min, which is accurate and able to trigger an alarm signalwithin a reasonable time.

A further object of the present invention is to provide a monitoringsystem for a dosage pump for low flow rates which is sturdy, andscarcely affected by the surroundings, such as ambient temperatures.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects havenow been realized by the invention of apparatus for monitoring the flowof a fluid through a dosage pump having a suction stroke for drawing thefluid into the dosage pump from a source of the fluid and a dischargestroke for discharging the fluid from the dosage pump, the apparatuscomprising an auxiliary pump disposed between the fluid source and thedosage pump, a slave chamber disposed between the auxiliary pump and thedosage pump, the slave chamber including a level detector for detectinga first predetermined level of the fluid in the slave chamber andemitting a signal when the level of the fluid in the slave chamber isbelow the first predetermined level, and control means for activatingthe auxiliary pump after the level detector emits the signal whereby theslave chamber is refilled with the fluid by the auxiliary pump after thesuction stroke of the dosage pump has drawn the fluid into the dosagepump and caused the level indicator to emit the signal. In a preferredembodiment, the dosage pump is incorporated in a dialyzer.

In accordance with one embodiment of the apparatus of the presentinvention, the control means is adapted to activate the auxiliary pumpto refill the slave chamber to a second predetermined level above thefirst predetermined level of the fluid in the slave chamber.

In accordance with another embodiment of the apparatus of the presentinvention, the apparatus includes regulating means for regulating thedosage pump whereby the suction stroke is carried out at a first speedand the discharge stroke is carried out at a second speed, the firstspeed being substantially greater than the second speed and the secondspeed providing a substantially constant flow rate.

In accordance with another embodiment of the apparatus of the presentinvention, the auxiliary pump includes measuring means for measuring thevolume of the fluid pumped by the auxiliary pump during each cyclethereof, the control means including time measuring means for measuringthe time between each cycle of the auxiliary pump and calculating meansfor calculating the flow of the fluid through the dosage pump based onthe ratio between the volume of the fluid measured by the measuringmeans and the time between each of the cycles of the auxiliary pumpmeasured by the time measuring means.

In accordance with another embodiment of the apparatus of the presentinvention, the auxiliary pump comprises a second dosage pump having apredetermined volume per cycle or portion thereof, and wherein the slavechamber includes a side wall and an outlet for the second dosage pump,the inlet being disposed adjacent to the side wall of the slave chamber.

In accordance with the present invention, a method has also been devisedfor monitoring the flow of a fluid through a dosage pump having asuction stroke for drawing the fluid into the dosage pump from a sourceof the fluid and a discharge stroke for discharging the fluid from thedosage pump, an auxiliary pump disposed between the source of the fluidand the dosage pump, and a slave chamber disposed between the auxiliarypump and the dosage pump, the method comprising detecting the level ofthe fluid in the slave chamber and emitting a signal when the level isbelow a first predetermined level in the slave chamber, and actuatingthe auxiliary pump after emitting the signal whereby the level of thefluid in the slave chamber is increased above a second predeterminedlevel by the discharge stroke of the dosage pump. In a preferredembodiment, the dosage pump is incorporated in a dialyzer.

In accordance with one embodiment of the method of the presentinvention, the second predetermined level is greater than the firstpredetermined level, whereby the slave chamber is topped up with apredetermined hysteresis value above the first predetermined level.

In accordance with another embodiment of the method of the presentinvention, the method includes activating the auxiliary pump with apredetermined time delay after emitting the signal.

In accordance with another embodiment of the method of the presentinvention, the method includes regulating the dosage pump so that thesuction stroke is carried out at a first speed and the discharge strokeis carried out at a second speed, the first speed being substantiallygreater than the second speed, and the second speed providing asubstantially constant flow rate.

In accordance with another embodiment of the method of the presentinvention, the method includes measuring the volume of the fluid flowingthrough the auxiliary pump for each cycle thereof, measuring the timebetween each cycle of the auxiliary pump, and calculating the fluid flowthrough the dosage pump by determining the ratio between the measuredvolume of the fluid flowing through the auxiliary pump and the measuredtime between each cycle of the auxiliary pump.

According to the present invention, the above objects are achieved by amethod and a system for monitoring a dosage pump, particularly in adialysis machine, in which the dosage pump has a suction stroke fordrawing a fluid into the dosage pump from a source of the fluid, and adischarge stroke for discharging the fluid from the dosage pump. Thesystem includes a second pump arranged between the source of the fluidand the dosage pump, a slave chamber arranged between the dosage pumpand the second pump, a level detector arranged in the slave chamber foremitting a signal when the level of the fluid in the slave chamber isbelow the level of the level detector, and a control arrangement foractivating the second pump when the level detector emits a signal duringand/or after a suction stroke of the dosage pump for refilling the slavechamber to a predetermined level before initiation of the next suctionstroke.

Preferably, the control arrangement according to the present inventionis arranged to fill the slave chamber with a predetermined volume abovethe level of the level detector. Moreover, there is preferably aregulating arrangement for regulating the dosage pump for obtaining afast suction stroke and a regulated discharge stroke in which thedischarge fluid flow rate is substantially constant.

In accordance with one embodiment of the present invention, the secondpump comprises a measuring arrangement for measuring the volume of fluidpassing through the second pump for each cycle, whereby the controlarrangement comprises a time measurement device for measuring the timebetween successive cycles, and a calculation arrangement for calculatingthe fluid flow rate through the dosage pump by determining the ratiobetween said volume measurement and said time measurement.

Preferably, the second pump is a dosage pump with a known volume perrevolution, or per partial revolution or the like, and the slave chamberconsists of a chamber with an inlet from the second pump, which inlet isarranged immediately adjacent the side wall of the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully appreciated with reference tothe following detailed description, which, in turn, refers to thedrawings in which:

FIG. 1 is a schematic diagram of a dialysis fluid preparation portion ofa dialysis machine provided with a monitoring system according to thepresent invention;

FIG. 2 is a top, elevational view of two pumps and an intermediate slavechamber in the monitoring system according to FIG. 1;

FIG. 3 is a top, elevational view of an alternative embodiment of theslave chamber of the present invention;

FIG. 4 is a front, elevational, partially sectional view of anotherembodiment of the slave chamber of the present invention; and

FIG. 5 is a front, elevational, partially sectional view of anotherembodiment of the slave chamber of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described below in greater details withreference to a preferred embodiment which is intended for use in adialysis machine, in particular, a GAMBRO AK 200, which is sold byGAMBRO AB of Sweden. The principles underlying the present invention maybe employed in other types of dialysis machines with modifications whichwill be readily apparent to skilled persons in the art.

A flow diagram is shown in FIG. 1 for the above-mentioned dialysismachine in which only that part of the equipment is shown which isrelevant to the present invention, namely the part in which preparationof the dialysis fluid takes place.

The dialysis machine is connected to an outlet for pure water which isnormally available in a dialysis clinic. The water normally comes froman RO-unit (reverse osmosis unit) and is substantially free of ions andother impurities.

The water enters a main conduit 1 in the dialysis machine according toFIG. 1 through an inlet conduit 2. The inlet conduit 2 opens into awater reservoir 3, in which the water is heated to the appropriatetemperature for use, which is normally about 37° C.

During a normal dialysis treatment, which lasts for about four hours,about 120 liters of water are used. Thus, it is necessary that ½ literof dialysis fluid be prepared per minute (500 ml/minute). Other dialysisfluid flow rates may be used, though they normally lie in the region ofabout 300 to 700 ml/minute.

The water from the water reservoir 3 flows along a conduit 4 and reachesa first dosage point 5 where a first concentrate is added to the mainflow, normally an A-concentrate. In addition, a second dosage point 6 isprovided where a B-concentrate is added. A first dosage pump 11 isconnected to the first dosage point 5 and a second dosage pump 12 isconnected to the second dosage point 6.

The dosage pumps, 11 and 12, are connected to sources for concentratesthrough conduits, 19 and 25. In the shown embodiment, cartridges areused containing sodium chloride powder 16 and sodium bicarbonate powder22. Water passes through the powder beds and forms concentrates of thesesubstances.

In addition, a small bag 27 is provided, hereinafter called an ionicbag, which comprises about ½ liter of fluid with the remainingconstituents which are not provided from the powder cartridges. Theionic bag 27 is arranged in a holder 28. A conduit 29 leads from theionic bag to a third concentrate pump or dosage pump 30. The dosage pump30 pumps the contents from the ionic bag through a conduit 31 whichopens into the conduit 19.

The dosage pump 30 pumps the fluid in the ionic bag at a rate which isdetermined by the computer equipment of the dialysis machine. Normally,the dosage pump 30 is a so-called ceramic pump of the piston type.During a suction stroke, suction of fluid into the pump chamber of thepump takes place. During a discharge stroke, discharge of the fluidtakes place, whereby the dosage pump is operated so that the suctionstroke is as short as possible and the discharge stroke is dimensionedso that the discharged flow rate is relatively constant. The pump isdriven by a step motor by means of a computer 36 in which theabove-mentioned functions are programmed.

The monitoring of the dosage pump 30 takes place according to thepresent invention by means of an identical or similar second pump 32 andan intermediate slave chamber 33 which are located in the conduit 29from the ionic bag to the dosage pump 30, as is shown in FIG. 1.

The fluid from the ionic bag 27 passes through the conduit 29 to thesecond pump 32 and further to the slave chamber 33 and to the dosagepump 30. As is apparent from FIG. 1, the upper end of the slave chamberis connected by a conduit 34 to the atmosphere by means of a connection18 for the purpose of deaeration. In this manner, any air bubbles in thefluid from the ionic bag are separated out before dosage, which resultsin the dosage pump performing better.

The monitoring arrangement functions as follows. The dosage pump 30 isdriven by a step motor which is controlled by the computer 36. Thecomputer 36 is programmed to drive the step motor and the dosage pump 30during the discharge stroke, during the first 180 degrees of rotation ofthe step motor, with a speed which results in the flow rate after thedosage pump being substantially constant, which means that the angularvelocity of the step motor is high in the beginning and at terminationof the half revolution, and is slow in the middle of the halfrevolution.

A suction stroke then takes place, in which liquid is fed to the dosagepump from the slave chamber. This suction stroke takes place as quicklyas possible, which means that the step motor is driven with high speedduring the second half revolution. This high speed is limited by suchconditions as back pressure, viscosity of the liquid, etc. If the speedis too great, there is a risk that the pump chamber of the dosage pumpis not completely filled during a suction stroke. If the speed is toolow, the period of interruption in the flow from the dosage pump becomestoo long.

The slave chamber 33 is provided with a level detector 35, as is moreclearly apparent from FIG. 2. During the suction stroke to the dosagepump, the level in the slave chamber drops below the level detector 35and this emits a signal. The signal activates the second pump 32 whichstarts to pump liquid to the slave chamber from the ionic bag 27. Therelationship between the flow from the slave chamber to the dosage pumpduring its suction stroke and the flow to the slave chamber from thesecond pump is such that the level in the slave chamber never reachesabove the level detector. This normally means that the flow to thedosage pump is always greater than the flow from the second pump to theslave chamber, if both pumps are driven simultaneously.

When the suction stroke to the dosage pump is terminated, i.e. when thestep motor has rotated from 180 degrees to 360 degrees, the flow fromthe slave chamber to the dosage pump becomes zero. Since the leveldetector 35 still emits a signal, the second pump continues to be drivenuntil the level in the slave chamber reaches the level detector.Thereafter, the second pump 32 is driven for a further short perioduntil a predetermined volume of liquid has been pumped to the slavechamber. The level in the slave chamber thus rises by a predeterminedvolume above the level detector. In this manner, a hysteresis isobtained in the signal of the level detector.

By means of this arrangement of the second pump and the slave chamber,the flow through the dosage pump 30 can be monitored. The flow throughthe second pump is known since this second pump is also a dosage pump,for example of the same type as the dosage pump 30. The time for eachcycle can be obtained from the electrical signal from the leveldetector. The flow is thus the ratio between the pump volume and thecycle time.

The conditions which are to be monitored are, among others, thefollowing:

1) that the flow from the dosage pump to the conduit 19 is always withincertain error tolerance limits, such as +/−5%;

2) that no leakage is present in the conduits so that the dosage pumpdoes not pump air;

3) that no cavitation takes place during the suction stroke to thedosage pump, which would result in a reduction of the displacement ofthe dosage pump, or that the dosage pump does not suffer a mechanical orelectrical malfunction;

4) that an individual fault in the circuit from the ionic bag 29 to theinput conduit 31 is detected, such as if no ionic bag is connected, theionic bag is empty, there is a blockage, either completely or partially,or a small hole or a leakage in the ionic bag, the connection or thetubes;

5) that there is a fault in the operation of the second pump, such asthat it operates at the wrong speed, misses steps or rotates in thewrong direction;

6) that there is a fault in the slave chamber, such as blockage orleakage in the slave chamber or in the deaeration conduit; and/or

7) that there is a fault in the level detector, such as constantly high,low, inverted or oscillating signal.

By employing a slave chamber, it is ensured that a condition in whichonly air passes through the dosage pump is detectable, resulting in thefact that no signal is obtained from the level detector.

By using a second pump in which the flow to the slave chamber may bemonitored and measured completely independently of the operatingfunction of the dosage pump, the dosage pump may be monitored so that itis within the stated range of +/−5%, or in the desired range.

If a malfunction arises, such as that a leakage arises in the conduitbetween the slave chamber and the dosage pump such that the dosage pumpdraws in air, this malfunction is detected within a stroke of the dosagepump due to the level detector not emitting a signal.

If a sudden change takes place in the delivery speed of the dosage pump,for example due to dramatically increased back pressure in the conduit31, an indication from the second pump is obtained which, during thenext stroke, pumps considerably less liquid or, alternatively, the timesignal for the level detector becomes different.

The monitoring arrangement comprising the second pump and the leveldetector is operated according to a particular algorithm.

The algorithm for the monitoring arrangement not only monitors thevolume which is pumped by the second pump 32, but it also relates thisvolume to a suitable time interval.

By relating the volume pumped by the second pump 32 to a period of thedosage pump 30, a stable and correct estimation with regard to theexpectation value of the flow through the dosage pump is obtained.

The dosage pump 30 has a stable period, typically +/−200 ms at a periodof about 11 seconds.

A cycle of the complex system of the dosage pump 30, the second pump 32and the slave chamber 33 can be described as follows:

When the dosage pump 30 initiates its suction period, the level in theslave chamber 33 drops quickly from a well determined level above thelevel of the level detector.

After a short but stable delay from the dosage pump 30 initiating itssuction stroke, the liquid level in the slave chamber 33 passes thelevel detector 35. When this is registered by the detector, themonitoring computer 36 terminates the time determination of the previouscycle of the dosage pump 30 and commences the next cycle. Since thedelay is stable and does not vary to any great extent from one cycle tothe other, the resulting time determination will provide a stableresult.

The monitoring computer 36 thereafter introduces a time delay duringwhich it calculates a flow rate estimation based on estimations of thepreceding period's pumped volume and period time. Thereafter, themonitoring computer 36 starts refilling the slave chamber 33. This delaydoes not affect the performance of the system as long as refilling iscompleted before the next suction stroke is initiated.

In the meantime, the dosage pump 30 has continued its suction stroke.When the second pump 32 starts to pump, this takes place at a flow ratewhich is lower than that which the dosage pump 30 has during its suctionperiod. Thus, the level in the slave chamber 33 will be a monotonouslydiminishing function of time. There is thus no way in which the level inthe slave chamber during the suction stroke of the dosage pump can crossthe detector 35.

The second pump 32 continues to fill the slave chamber 33 and, when thedosage pump 30 changes from suction stroke to discharge stroke, thelevel in the chamber rises.

When the level passes the level of the detector, the detector emits asignal and the monitoring computer 36 reacts by calculating how muchlonger filling should continue. Should the detector emit a plurality ofclosely spaced signals, the monitoring computer 36 will perform a newcalculation of the continued filling time for each new signal. Thus, thefilling time will be resistant to possible interference due to aplurality of successive signals from the level detector. If the secondpump 32 is a ceramic dosage pump with a step motor, the calculation ofcontinued filling time will depend on where on the rotation the detectorsignal is received. The continued filling takes place in order toachieve a constant, predetermined hysteresis volume above the switchinglevel of the sensor. The hysteresis volume reduces the risk of falsedetector signals.

When the hysteresis volume is obtained, the monitoring computer 36 stopsthe second pump 32 and calculates the pumped volume. The dosage pump 30is now in its relatively long discharge period, while the monitoringcomputer 36 awaits the next suction stroke and the level detector signaloccasioned thereby.

An alternative embodiment of the slave chamber is shown in FIG. 3. Theslave chamber consists of two portions, a first lower portion 40 oflarge cross-section and a second upper cylindrical portion 41 of narrowcross-section, the upper end of which is connected to the ambientpressure by means of the conduit 34. The level detector 35 is positionedat the upper cylindrical narrow portion. Because of the above-mentionedhysteresis effect, the liquid level in the slave chamber, 40 and 41, isabove the level detector 35 in the narrow portion, as indicated by theline 42 in FIG. 3.

During the dwell position shown in FIG. 3, liquid is continuouslydischarged from the dosage pump 30 to the conduit 31 while the flow tothe dosage pump 30 is zero, i.e. the step motor for the dosage pumpoperates between 0 degrees and 180 degrees. This is indicated by thearrow 51.

When the step motor rotates past 180 degrees, the discharge becomes zeroand suction of liquid to the dosage pump takes place instead, asindicated by the arrow 52. This results in the level 42 dropping in theslave chamber, 40 and 41, as indicated by the arrow 53. When the level42 drops below the level detector 35, a signal is emitted whichactivates the pump 32, as indicated by the arrow 54, though with acertain time delay. When the suction stroke to the dosage pump has beencompleted, i.e. when the step motor rotates past 360 degrees, the flowaccording to the arrow 52 becomes zero and the level increases in theslave chamber, 40 and 41, until it rises above the level detector 35.Finally, a top-up of a predetermined hysteresis volume takes place, asindicated by the hatched lines in FIG. 3.

As mentioned above, the second pump 32 is activated with a certain timedelay. The reason for this is that it is necessary to ensure that thedosage pump does in fact pass 360 degrees before the level once morereaches above the level detector 35. The surest way to achieve this isto ensure that the second pump 32 has a lower or substantially the sameflow rate as the dosage pump and it is started a short time after thelevel has dropped below the level detector.

It is possible to permit the above-mentioned time delay to be so greatthat the second pump 32 is initially started after the dosage pump hasrotated past 360 degrees, i.e. the suction stroke is completed and thedischarge stroke recommences.

For reasons of safety, the lower portion 40 of the slave chamber isdimensioned so as to have a volume which is as great as, or larger than,the displacement of the dosage pump 30.

By providing for a top-up volume above the level of the level detector,several advantages are attained.

It is possible to prevent the level detector from stopping the refillingbefore the level actually reaches the level detector due to temporarydisturbances, such as the level detector temporarily emitting a signal,for example because the inlet flow affects the level measurement. Ifsuch influenced signals arise, the last signal which was obtained isused to calculate the hysteresis volume.

In addition, it is possible to obtain a very distinct determination ofthe cycle time. When a suction stroke for the dosage pump commences,i.e. when the dosage pump rotates past 180 degrees, a rapid and distinctdrop of the level in the slave chamber past the level detector takesplace. The flow rate is greatest at the middle of the suction stroke,i.e. about 270 degrees. If the slave chamber's hysteresis volume is thusabout one half the displacement of the dosage pump, a signal is obtainedfrom the level detector when the flow velocity is greatest, therebyimplying that the change-over is distinct. In addition, the second pumpis inactive until such a change-over is obtained, i.e. the second pumpdoes not interfere with the determination of the cycle time.

Since the cycle time is determined by the time between two successivesignals when the level in the slave chamber drops below the leveldetector, the time determination becomes as accurate as possible. Thisis of considerable importance for the monitoring system to operatesatisfactorily. An inaccuracy in the time determination is reflected inthe denominator in the flow determination, which results in a non-linearfunction which is not easy to rectify.

In a preferred prototype of the present invention, a dosage pump of theceramic type is used having a displacement of about 250 μl, a dischargestroke which lasts about 11 seconds and a suction stroke which lasts afew tenths of a second. A second pump of the same construction is alsoused. The hysteresis volume, i.e. the hatched region in FIG. 3, has avolume of about 125 μl.

A further alternative embodiment of the slave chamber according to thepresent invention is shown in FIG. 4. The slave chamber 60 has an inletconduit 61 from the second pump 32 and an outlet conduit 62 to thedosage pump 30. At least the inlet conduit 61 is arranged tangentially.In this manner, the fluid flows in through the inlet conduit 61 over theinner surface of the slave chamber 60 along the surface and down untilthe fluid reaches the level of the slave chamber. By means of thisarrangement of the inlet conduit, it is ensured that the level detector35 reacts only when the level of the fluid in the chamber is above thelevel detector without being affected by intake of fluid through theinlet conduit.

Yet another alternative embodiment of the slave chamber is shown in FIG.5. The slave chamber consists of a tube 70 corresponding to the chamber60 in FIG. 4, whereby only the upper portion of the tube 70 is shown inFIG. 5. A level detector 35 is shown in the form of a transmitter 71 anda receiver 72 for infra-red radiation. An insert 73 is positioned in thetube 70. A seal (not shown in FIG. 5) is located between the upper endof the tube 70 and the insert. The insert 73 comprises a deaeration tube74 corresponding to the conduit 34 in FIG. 1 and a feed-tube 75 whichsupplies liquid to the second pump. The feed-tube 75 discharges belowthe deaeration tube 74 and directly onto the wall of the tube 70 so thatthe supplied fluid runs across the wall of the tube to the liquid levelin the tube 70 and thereby affects the level detector as little aspossible. It is to be understood that the tube 70 and the insert 73 canbe manufactured in one piece, whereby the feed-tube 75 can be chamferedso that an even better fit to the inner wall of the tube is obtained.

The deaeration conduit, 74 and 34, exhausts to the atmosphere.Alternatively, it is possible to permit the conduit, 74 and 34, to leadto some other container, for example a pressure equalization containeror an expandable container.

It is possible within the scope of the present invention to use a secondpump having a different construction from the concentrate dosage pump,since it is only necessary that the second pump is of the metering pumptype, i.e. the flow rate has a particular relationship to the number ofrevolutions or partial revolutions of the drive motor.

The second pump serves to fill the slave chamber so that it is neverempty and to measure the volume which is necessary to fill the chamberduring each cycle. Any pump or construction which satisfies this purposecan be used as an alternative to the second pump.

The monitoring of the dosage pump takes place by means of a monitoringcomputer 36. According to a preferred embodiment of the presentinvention, the monitoring computer 36 is arranged to receive itsinformation from the second pump. The second pump is provided with arevolution indicator which indicates when the pump shaft is at aninitial position, for example 0 degrees. If the revolution indicatordoes not emit a signal within a predetermined time interval which isrelated to the filling volume of the slave chamber, an alarm signal isemitted.

In the preferred embodiment, the second pump is of the dosage pump typeand is driven by a step motor. The monitoring computer 36 detects theposition of the step motor and determines the volume which has been fedto the slave chamber during one cycle. By dividing the input volume bythe cycle time, an estimation of the flow for the dosage pump isobtained. If this estimation lies outside of predetermined limits, analarm signal is emitted.

A malfunction which must be taken into account is if the controlarrangement partially ceases to operate. It is, for example, conceivablethat it fails to refill the slave chamber while it continues to send outthe last, and accepted, flow estimation to the monitoring system. Insuch a case, the dosage pump would pump air while the second pump'scontrol arrangement would indicate that everything was operating asintended.

A manner in which such a malfunction can be detected is the following:The second pump has a rotation detector which emits one pulse perrevolution. These pulses are normally registered by the controlarrangement and a copy of the computation value is sent for each newpulse to the monitoring computer 36 of the dialysis machine. In themonitoring computer 36, the time interval between the rotation pulses isused to provide a rough flow estimation. If no pulses arrive at themonitoring computer 36, after, for example, one minute it will determinethe rough estimation as zero. As long as the flow estimation sent by thecontrol arrangement is sufficiently close, for example +20%, to thecoarse flow estimation, the monitoring computer 36 will use the firstmentioned estimation. Otherwise, the latter estimation is used.

The present invention has been described above with reference topreferred embodiments of the invention. The various properties andfeatures which have been described can be combined in other ways thanthose which have been described in relation to the embodiments. Suchmodifications which are evident to a skilled person are intended to fallwithin the scope of the invention. The invention is limited solely bythe appended claims.

What is claimed is:
 1. Apparatus for monitoring the flow of a fluidthrough a dosage pump having a suction stroke for drawing said fluidinto said dosage pump from a source of said fluid and a discharge strokefor discharging said fluid from said dosage pump, said dosage pumpincorporated as an internal part of a dialysis machine to regulate theamount of a pre-prepared dialysis fluid flowing through said dialysismachine to be mixed with a concentrate fluid simultaneously being pumpedthrough the dialysis machine, said apparatus comprising an auxiliarypump disposed between a fluid source and said dosage pump, a slavechamber disposed between said auxiliary pump and said dosage pump, saidslave chamber including a level detector for detecting a firstpredetermined level of said fluid in said slave chamber and emitting asignal when said level of said fluid in said slave chamber is below saidfirst predetermined level, and control means for activating saidauxiliary pump after said level detector emits said signal whereby saidslave chamber is refilled with said fluid by said auxiliary pump aftersaid suction stroke of said dosage pump has drawn said fluid into saiddosage pump and caused said level detector to emit said signal.
 2. Theapparatus of claim 1 wherein said control means is adapted to activatesaid auxiliary pump to refill said slave chamber to a secondpredetermined level above said first predetermined level of said fluidin said slave chamber.
 3. The apparatus of claim 1 including regulatingmeans for regulating said dosage pump whereby said suction stroke iscarried out at a first speed and said discharge stroke is carried out ata second speed, said first speed being substantially greater than saidsecond speed and said second speed providing a substantially constantflow rate.
 4. The apparatus of claim 1 wherein said auxiliary pumpincludes measuring means for measuring the volume of said fluid pumpedby said auxiliary pump during each cycle thereof, said control meansincluding time measuring means for measuring the time between each cycleof said auxiliary pump and calculating means for calculating the flow ofsaid fluid through said dosage pump based on the ratio between saidvolume of said fluid measured by said measuring means and said timebetween each of said cycles of said auxiliary pump measured by said timemeasuring means.
 5. The apparatus of claim 1 wherein said auxiliary pumpcomprises a second dosage pump having a predetermined volume per cycleor portion thereof, and wherein said slave chamber includes a side walland an outlet for said second dosage pump, said inlet being disposedadjacent to said side wall of said slave chamber.
 6. A method formonitoring the flow of a fluid through a dosage pump having a suctionstroke for drawing said fluid into said dosage pump from a source ofsaid fluid and a discharge stroke for discharging said fluid from saiddosage pump, said dosage pump incorporated as an internal part of adialysis machine to regulate the amount of a pre-prepared dialysis fluidflowing through said dialysis machine to be mixed with a concentratefluid simultaneously being pumped through the dialysis machine, anauxiliary pump disposed between said source of said fluid and saiddosage pump, and a slave chamber disposed between said auxiliary pumpand said dosage pump, said method comprising detecting the level of saidfluid in said slave chamber and emitting a signal when said level isbelow a first predetermined level in said slave chamber, and actuatingsaid auxiliary pump after emitting said signal whereby said level ofsaid fluid in said slave chamber is increased above a secondpredetermined level by said discharge stroke of said dosage pump.
 7. Themethod of claim 6 wherein said second predetermined level is greaterthan said first predetermined level, whereby said slave chamber istopped up with a predetermined hysteresis value above said firstpredetermined level.
 8. The method of claim 6 including activating saidauxiliary pump with a predetermined time delay after emitting saidsignal.
 9. The method of claim 6 including regulating said dosage pumpso that said suction stroke is carried out at a first speed and saiddischarge stroke is carried out at a second speed, said first speedbeing substantially greater than said second speed, and said secondspeed providing a substantially constant flow rate.
 10. The method ofclaim 6 including measuring the volume of said fluid flowing throughsaid auxiliary pump for each cycle thereof, measuring the time betweeneach cycle of said auxiliary pump, and calculating the fluid flowthrough said dosage pump by determining the ratio between said measuredvolume of said fluid flowing through said auxiliary pump and saidmeasured time between each cycle of said auxiliary pump.
 11. Apparatusfor monitoring the flow of a fluid through a dosage pump having asuction stroke for drawing said fluid into said dosage pump from asource of said fluid and a discharge stroke for discharging said fluidfrom said dosage pump, said apparatus comprising an auxiliary pumpdisposed between a fluid source and said dosage pump, a slave chamberdisposed between said auxiliary pump and said dosage pump, said slavechamber including a level detector for detecting a first predeterminedlevel of said fluid in said slave chamber and emitting a signal whensaid level of said fluid in said slave chamber is below said firstpredetermined level, control means for activating said auxiliary pumpafter said level detector emits said signal whereby said slave chamberis refilled with said fluid by said auxiliary pump after said suctionstroke of said dosage pump has drawn said fluid into said dosage pumpand caused said level detector to emit said signal, and regulating meansfor regulating said dosage pump whereby said suction stroke is carriedout at a first speed and said discharge stroke is carried out at asecond speed, said first speed being substantially greater than saidsecond speed, and said second speed providing a substantially constantflow rate.
 12. Apparatus for monitoring the flow of a fluid through adosage pump having a suction stroke for drawing said fluid into saiddosage pump from a source of said fluid and a discharge stroke fordischarging said fluid from said dosage pump, said apparatus comprisingan auxiliary pump disposed between a fluid source and said dosage pump,said auxiliary pump comprising a second dosage pump having apredetermined volume per cycle or portion thereof, a slave chamberdisposed between said auxiliary pump and said dosage pump, said slavechamber including a level detector for detecting a first predeterminedlevel of said fluid in said slave chamber and emitting a signal whensaid level of said fluid in said slave chamber is below said firstpredetermined level, said slave chamber further including a side walland an inlet for said second dosage pump, said inlet being disposedadjacent to said side wall of said slave chamber, and control means foractivating said auxiliary pump after said level detector emits saidsignal whereby said slave chamber is refilled with said fluid by saidauxiliary pump after said suction stroke of said dosage pump has drawnsaid fluid into said dosage pump and caused said level detector to emitsaid signal.
 13. A method for monitoring the flow of a fluid through adosage pump having a suction stroke for drawing said fluid into saiddosage pump from a source of said fluid and a discharge stroke fordischarging said fluid from said dosage pump, an auxiliary pump disposedbetween said source of said fluid and said dosage pump, and a slavechamber disposed between said auxiliary pump and said dosage pump, saidmethod comprising detecting the level of said fluid in said slavechamber and emitting a signal when said level is below a firstpredetermined level in said slave chamber, actuating said auxiliary pumpafter emitting said signal whereby said level of said fluid in saidslave chamber is increased above a second predetermined level by saiddischarge stroke of said dosage pump, and regulating said dosage pump sothat said suction stroke is carried out at a first speed and saiddischarge stroke is carried out at a second speed, said first speedbeing substantially greater than said second speed, and said secondspeed providing a substantially constant flow rate.